1. Field of the Invention
The present invention relates to a level control circuit for executing automatic gain control (AGC) and clamp of a video signal in a magnetic recording/playback apparatus or the like and, more particularly, to a sync AGC circuit for adjusting the sync level of a video signal to a predetermined level.
2. Description of the Prior Art
The video signal AGC circuits known heretofore are classified into an analog type and a digital type.
FIG. 5 is a block diagram of a conventional analog video AGC circuit; FIG. 6 is a block diagram of a conventional digital video AGC circuit; and FIG. 7 is a waveform chart of a video signal inputted to a video AGC circuit.
In FIGS. 5 and 7, an analog gain control amplifier 21 is controlled by the output of a control voltage generator 24 to adjust the gain so that a sync level L, which is the difference between a sync tip level a of the output video signal and a pedestal level b thereof, is maintained at a predetermined level (e.g., 0.5 V). A sync level detector 22 detects the sync level. L of the video signal outputted from the analog gain control amplifier 21. This detector 22 consists of a circuit for sampling and holding the sync tip level a of the video signal and the pedestal level b thereof and outputting the difference. A reference level adjuster 23 generates a reference voltage for setting the sync level L of the output video signal to a predetermined level (e.g., 0.5 V), and a control voltage generator 24 generates a difference voltage between the sync level L of the output video signal obtained from the sync level detector 22 and the reference voltage outputted from the reference level adjuster 23, and then delivers the gain control voltage to the analog gain control amplifier 21.
In the analog video AGC circuit of the configuration mentioned above, feedback control is so performed that the sync level L of the output video signal obtained from the sync level detector 22 is equalized to the output of the reference level adjuster 23, whereby the sync level L of the output video signal is maintained under control at a predetermined level (e.g., 0.5 V).
Next in FIG. 6, an analog gain control amplifier 31 is controlled by the output of a control voltage generator 36 and adjusts the gain for equalizing the sync level L of the output video signal to a predetermined level (e.g., 0.5 V). An A-D converter 32 converts the output of the analog gain control amplifier 31 to, e.g., a 10-bit parallel digital video signal. A sync signal detector 33 detects a sync signal included in the digital video signal, and a pedestal detector 34 detects pedestal data, which corresponds to the pedestal level b of the digital video signal, on the basis of the output of the sync signal detector 33. A sync tip detector 35 detects sync tip data, which corresponds to the sync tip level a of the digital video signal, on the basis of the output of the sync Signal detector 33. Further a control voltage generator 36 generates a gain control voltage on the basis of the pedestal data obtained from the pedestal level detector 34 and also on the basis of the sync tip data obtained from the sync tip detector 35, so as to equalize the sync level L of the output digital video signal, which is the difference between the pedestal data and the sync tip data, to a predetermined level (e.g., 0.5 V). And such gain control voltage is supplied to the analog gain control amplifier 31.
In the digital video AGC circuit of the configuration mentioned above, the sync level of the output digital video signal is controlled to a predetermined level (e.g., 0.5 V) as in the foregoing case of FIG. 5. And the operation of clamping the video signal is performed usually in the preceding stage of the analog gain control amplifier 21.
However, in such conventional analog video AGC circuit, there exists a problem that a nonadjustment constitution is not realizable due to the necessity of a reference level adjuster. Furthermore, a high-resolution gain control amplifier is required to execute the gain control with a high precision to consequently render the circuit operation unstable.
Since the conventional video AGC circuit mentioned above has a feedback control configuration, oscillation is caused if the response speed is raised, and the transient response is unsatisfactory at the occurrence of any sharp variation in the input video signal level or at the rise from a no-signal stage. And there are some more problems including that the gain of the analog gain control amplifier is controlled to its maximum at the rise from a no-signal state, and proper detection of the sync signal fails to be performed. And upon simultaneous occurrence of such undesired conditions, a release from the above state is rendered impossible or needs an extremely long time.
Furthermore, due to the constitution of hardware alone, it is impossible to achieve a variety of settings for individual cases. Therefore, the compliance is limited merely to one for various input conditions of video signals where, for example, the ratios of the sync levels to the peak-to-peak values of the video signals are mutually different.